1. Field of the Invention
The present invention relates to global positioning system (GPS) receivers operable for navigating and tracking vehicles. More particularly, the invention relates to a method and apparatus for correcting errors in the receiver's code phase measurement caused by clock errors introduced during downconversion of the received satellite signals.
2. Description of the Prior Art
GPS satellites include highly accurate on-board atomic clocks for timing the transmission of their navigation and code signals. GPS receivers receive the satellite signals, downconvert the received signals to intermediate frequency signals, generate replica signals to track or match the downconverted signals, and shift the phase of the generated replica signals until a match or phase lock is obtained. Once phase lock is obtained, the receiver reads the navigation signals to calculate the Doppler frequency of the GPS receiver relative to the satellite.
Unfortunately, the reference clocks on GPS receivers are not as accurate as the atomic clocks on-board the GPS satellites. Therefore, these reference clocks introduce clock errors into the code phase matching, especially during the downconversion of the received code signals.
Known prior art GPS receivers compensate for these reference clock errors by measuring the carrier phase errors with phase lock loop software and periodically adjusting the phase of the replica code generator to eliminate the measured errors. These prior art methods are limited, however, because they do not continuously compensate for the clock errors as they occur. Instead, the phase errors are only corrected after the accumulated phase difference reaches a predetermined value. Thus, prior art GPS receivers do not adequately compensate for reference clock errors, and as a result, often cannot achieve high precision tracking and navigation.